Soil chemistry

Soils are complex chemical factories. Regardless of the type of soil, chemical processes such as plant growth, organic decay, mineral weathering, and water purification are ongoing processes.

Weathering Process

Soil chemistry has been studied as long as there has been sustainable agriculture. Although they did not recognize it as such, those first successful farmers who plowed under plant stalks, cover crops, or animal wastes were actively managing the soil chemistry of their fields. These early farmers knew that to have productive farms in one location season after season, they had to return something to the soil. It is now understood that soil chemistry is a complex of chemical and biochemical reactions. The most obvious result of this complex of reactions is that some soils are very fertile whereas other soils are not. Soil itself is a unique environment because all of the “spheres”—the atmosphere, hydrosphere, geosphere, and biosphere—are intimately mixed there. For this reason, soil and soil chemistry are extremely important.

Soil chemistry begins with rock weathering. The minerals making up a rock exposed at the earth’s surface are continually bathed in a shower of mildly acidic rain—not polluted rainwater but naturally occurring acid rain. Each rain droplet forming in the atmosphere absorbs a small amount of carbon dioxide gas. Some of the dissolved carbon dioxide reacts with the water to form a dilute solution of carbonic acid. A more concentrated solution of carbonic acid is found in any bottle of sparkling water.

Most of the common rock-forming minerals, such as feldspar, will react slowly with rainwater. Some of the chemical elements of the mineral, such as sodium, potassium, calcium, and magnesium, are very soluble in rainwater and are carried away with the water as it moves over the rock surface. Other chemical elements of the mineral, such as aluminum, silicon, and iron, are much less soluble. Some of these elements are dissolved in the water and carried away; most, however, remain near the original weathering, where they recombine into new, more resistant minerals. Many of the new minerals are of a type called clays.

Clay minerals tend to be very small crystals composed of layers of aluminum and silicon combined with oxygen or hydroxyl ions. Between the layers of aluminum and silicon atoms are positively charged ions (cations) of sodium, potassium, calcium, and magnesium. The cations hold the layers of some clays together by electrostatic attraction. In most cases, the interlayer cations are not held very tightly. They can migrate out of the clay and into the water surrounding the clay mineral, to be replaced by another cation from the soil solution. This phenomenon is called cation exchange.

The weathering reactions between rainwater and rock minerals produce a thin mantle of clay mineral soil. The depth to fresh, unweathered rock is not great at first, but rainwater continues to fall, percolating through the thin soil and reacting with fresh rock minerals. In this way, the weathering front (the line between weathered minerals and fresh rock) penetrates farther into the rock, and the overlying soil gets thicker.

Biological Processes

Throughout the weathering process, biological processes contribute to the pace of soil formation. In the very early stages, lichens and fungi are attached to what appear to be bare rock surfaces. In reality, they are using their own acids to “digest” the rock minerals. They absorb the elements of the mineral they need, simultaneously extending fine filaments into the rock for attachment, and the remainder is left to form soil minerals. As the soil gets thicker, larger plants and animals begin to colonize it. Large plants send roots down into the soil looking for water and nutrients. Some of the necessary nutrients, such as potassium, are available as exchangeable cations on soil clays or in the form of deeper, unweathered minerals. In either case, the plant obtains the nutrients by using its own weathering reaction carried on through its roots. The nutrient elements are removed from minerals and become part of the growing plant’s tissue.

Without a way to replenish the nutrients in the soil, the uptake of nutrients by plants will eventually deplete the fertility of the soil. Nutrients are returned to the soil through the death and decay of plants. Microorganisms in the soil, such as bacteria and fungi, speed up the decay. Since the bulk of the decaying plant material is found at the surface (the dead plant’s roots also decay), most of the nutrients are released to the surface layer of the soil. Some of the nutrients are carried down to roots deep in the soil by infiltrating rainwater. Most of the nutrients, however, are removed from the water by the shallow root systems of smaller plants. The deeper roots of typically large plants can mine the untapped nutrients at the deep, relatively unweathered soil-rock boundary.

The soil and its soil chemistry are now well established, with plants growing on the surface and their roots reaching toward mineral nutrients at depth. Water is flowing through the soil, carrying dissolved nutrients and the soluble by-products of weathering reactions. Yet the soil continues to evolve even after it is well established. Eventually, when the downward migration of the weathering front matches the rate at which soil is eroded from the surface, the soil reaches its maximum evolution. In tropical and subtropical climates, the end stage of soil evolution is very deep soil in which most of the nutrients are gone. Virtually all nutrients are contained in the vegetation, and people often assume, falsely, that such soils are extremely productive.

Study of Soil Water

The study of soil chemistry is concerned with the composition of soil water and how that composition changes as the water interacts with soil atmosphere, minerals, plants, and animals. Soil and its chemistry can be studied in its natural environment, or samples can be brought into the laboratory for testing. Some tests have been standardized and are best conducted in the laboratory so that they can be compared with the results of other researchers. Most standardized tests, such as measurements of the soil’s acidity and cation-exchange capacity, are related to measurements of the soil’s fertility and its overall suitability for plant growth. These tests measure average values for a soil sample because large original samples are dried and thoroughly mixed before smaller samples are taken for the specific test.

Increasingly, soil chemists are looking for ways to study the fine details of soil chemical processes. They know, for example, that soil water chemistry changes as the water percolates through succeeding layers of the soil. The water flowing through the soil during a rainstorm has a different chemical composition from that of water clinging to soil particles, at the same depth, several days later. Finally, during a rainstorm, the water flowing through large cracks in the soil has a chemical composition different from that of the same rainwater flowing through the tiny spaces between soil particles.

Sampling Techniques

Soil chemists use several sampling techniques to collect the different types of soil water. During a rainstorm, water flows under the influence of gravity. After digging a trench in the area of interest, researchers push several sheets of metal or plastic, called pan lysimeters, into the wall of the trench at specified depths below the surface. The pans have a very shallow V shape. Soil water flowing through the soil collects in the pan, flows toward the bottom of the V, and flows out of the pan into a collection bottle. Comparing the chemical compositions of rainwater that has passed through different thicknesses of soils (marked by the depth of each pan) allows the soil chemist to identify specific soil reactions with specific depths.

After the soil water stops flowing, water is still trapped in the soil. The soil water clings to soil particles and is said to be held by tension. Tension water can spend a long time in the soil between rainstorms. During that time, it reacts with soil mineral grains and soil microorganisms. Tension water is sampled by placing another type of lysimeter, a tension lysimeter, into the soil at a known depth. A tension lysimeter is like the nozzle of a vacuum cleaner with a filter over the opening. Soil chemists actually vacuum the tension water out of the soil and to the surface for analysis.

Determination of Isotopic Composition

Nonradioactive, stable isotopes of common elements are being used more often by soil chemists to trace both the movement of water through the soil and the chemical reactions that change the composition of the water. Trace stable isotopes behave chemically just the way their more common counterparts do. For example, deuterium, an isotope of hydrogen, substitutes for hydrogen in the water molecule and allows the soil chemist to follow the water’s movements. Similarly, carbon-13 and nitrogen-15 are relatively uncommon isotopes of biologically important common elements. Using these isotopes, soil chemists can study the influences of soil organisms on the composition of soil water. Depending on what the soil chemist is studying, the isotope may be added, or spiked, to the soil in the laboratory or in the field. Spiking allows the movement of the isotope to be tracked by following the unusual concentration of the isotope. Alternatively, naturally occurring concentrations of the isotope in rain or snowmelt may be used. Regardless, soil water samples are collected by one or more of the lysimeter methods, and their isotopic composition is determined.

The Soil Chemical Factory

The wonderful interactions of complex chemical and biochemical reactions that are soil chemistry are one indication of the uniqueness of planet Earth. Without the interaction of liquid water and the gases in the atmosphere, many of the nutrients necessary for life would have remained locked up in rock minerals. Thanks to weathering reactions, the soil chemical factory started to produce nutrients, which resulted in the exploitation of the soil environment by millions of organisms. The processes involved in soil chemistry—from weathering reactions that turn rock into new soil to the recycling of plant nutrients through microbial decay—are vital to every human being. Without fertile soil, plants will not grow. Without plants as a source of oxygen and food, there would be no animal life.

Because of the complex chemical interrelationships that have developed in the soil environment, it may seem that nothing can disrupt the “factory” operation. As more is understood about soil chemistry and the ways in which human activities stress soil chemistry, it is apparent that the factory is fragile. Not only do humans rely on soil fertility for their very existence, but they also are taking advantage of soil chemical processes to help them survive their own past mistakes. Soil has been and continues to be used as a garbage filter. Garbage, whether solid or liquid, has been dumped on or buried in soil for ages. Natural chemical processes broke down the garbage into simpler, less toxic or unsanitary forms and recycled the nutrients. When garbage began to contain toxic chemicals, those chemicals, when in small quantities, were either destroyed by soil bacteria or firmly attached to soil particles. The result is that water—percolating through garbage, on its way to the local groundwater, stream, or lake—does not carry with it as much contamination as one might expect. Soil chemistry has, so far, kept contaminated garbage from ruining drinking water in many cases. There are well-known cases, however, where the volume and composition of waste buried or spilled were such that the local soil chemistry was overwhelmed. In cases of large industrial spills, or when artificial chemicals are spilled or buried, the soil needs help to recover. The recovery efforts are usually very expensive but, faced with the possible long-term loss of large parts of the soil chemical factory, humankind cannot afford to neglect this aspect of the environment.

Principal Terms

cation: an element that has lost one or more electrons such that the atom carries a positive charge

clay mineral: a group of minerals, commonly the result of weathering reactions, composed of sheets of silicon and aluminum atoms

hydroxl: a combination of an oxygen and hydrogen atom, which behaves as a single unit with a negative charge

lysimeter: a simple pan or porous cup that is inserted into the soil to collect soil water for analysis

stable isotopes: atoms of the same element that differ by the number of neutrons in their nuclei yet are not radioactive

weathering: reactions between water and rock minerals at or near the earth’s surface that result in the rock minerals being altered to a new form that is more stable

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